opEquals for multiple types

[Jonathan M Davis]

On Sunday, July 08, 2012 10:56:25 Wouter Verhelst wrote:
> Hi,
> 
> I'm trying to implement opEquals. I had originally done something like
> 
> class foo : bar {
>       bool opEquals(bar e) {
>            ... code here ...
>       }
>       bool opEquals(foo e) {
>            ... somewhat more specialized code here ...
>       }
> }
> 
> (where bar is an interface)
> 
> but that gave me a message about how hiding Object.opEquals is
> deprecated.
> 
> My next attempt was to add a
> 
>       bool opEquals(Object o)
> 
> which would then decide which opEquals function to call based on what
> typeid() on the Object returns. But since I want to have a version
> involving an interface, that's not going to fly.
> 
> What's the best way forward?

For classes, opEquals _must_ take Object. It can't take anything else. If you 
want to do something differently for specific base classes, you're going to need 
to deal with it within opEquals via casting. However, that's _still_ not 
likely to do what you want, because D ensures that equality goes in _both_ 
directions, so if the types differ, opEquals must be true for both 
a.opEquals(b) and b.opEquals(a). You can't have only one side deciding that 
it's equal to the other. They have to agree.

- Jonathan M Davis